(a) Technical Field
The present invention relates to a separator for a fuel cell. More particularly, it relates to a separator for a fuel cell which can minimize gas diffusion and concentration differences between areas in a gas diffusion layer and achieve uniform electrochemical reaction and electricity generation over the whole reaction area, by improving the structure of a flow field in which reactant gases flow.
(b) Background Art
Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are employed to generate electricity by electrochemically reacting hydrogen and oxygen (or air) as reactant gases.
Since PEMFCs have high efficiency, high current density and power density, short startup time, and quick response characteristics with respect to load variation compared to other types of fuel cells, PEMFCs may be applied to various fields such as power sources for pollution-free vehicles and power sources for independent power generation, transport, and military.
Generally, fuel cells are used in a stacked form in which unit cells are stacked to satisfy a necessary power level. Since fuel cells mounted in vehicles also need high power, hundreds of unit cells are stacked to meet requirements.
A Membrane-Electrode Assembly (MEA) is disposed at the innermost portion of the unit cell structure of a fuel cell stack. The MEA includes a solid polymer electrolyte membrane and catalyst electrodes, i.e., anode and cathode, that are configured by coating catalyst on both surfaces of the electrolyte membrane.
A gas diffusion layer (hereinafter, referred to as GDL), a gasket, and the like are disposed outside the MEA, i.e., outside the anode and the cathode. Also, a separator is disposed outside the GDL to provide a flow field for supplying reactant gases and exhausting water generated from a reaction.
In such a structure, an oxidation reaction of hydrogen is performed in the anode of the fuel cell to generate protons and electrons. Protons and electrons that are generated are moved to cathode through the electrolyte membrane and the separator, respectively.
Thus, water is generated by an electrochemical reaction in which protons and electrons transferred from the anode and oxygen from air are involved, and heat is generated together with water during the electrochemical reaction. Also, electrical energy is generated from the flow of electrons.
On the other hand, the separator is a part that separates unit cells in the fuel cell stack and serves as a current passage between cells. The flow field formed in the separator serves as a supply passage for delivering reaction gases to GDL and an exhaust passage for exhausting water from GDL.
Examples of separators include graphite separators formed of graphite materials and metallic separators formed of metallic materials such as stainless steel. Recently, many studies are being conducted to replace graphite separators with metallic separators in consideration of workability and mass-production.
FIG. 1 is a cross-sectional view illustrating a typical metallic separator, in which a MEA 11, a GDL 12, and a separator 20 are boned to each other.
As shown in the drawing, the separator 20 includes a land (contacting) portion directly bonded to the GDL 12 and a channel portion that serves as a supply passage (passage of air and hydrogen) of reaction gases and an exhaust passage of water between land portions.
The channel portions of the typical separator 20 are disposed substantially parallel to each other over the whole of a reaction area of a fuel cell, or are disposed to form an inclined flow field. There are advantages and disadvantages in the characteristics such as cell performance, pressure, and water exhaust according to methods for designing the flow field, but methods for supplying reaction gases by processing a flow field of a rectangular sectional structure and other structures similar thereto at a portion corresponding to the reaction area are being commonly employed.
In such a separator, the land portion and the channel portion are disposed in a longitudinal direction, and thus an area in which the land portion is bonded to GDL and a flow field (supply passage of reaction gases and exhaust passage of water) area in which the channel portion is formed both have a longitudinal structure. Also, the land portion and the channel portion are distinctly separated from each other.
In this case, since the inner surface of the channel portion is smooth, the flow of reaction gases in the channel portion shows the characteristics of a laminar flow. Reaction gases are delivered and diffused to GDL by a pressure difference or a concentration difference without any force caused by flow while reaction gases flow along a long flow field.
Also, the diffusion amount of gases delivered to GDL due to a flow difference between the land portion and the channel portion varies according to a GLD area to which the land portion and the channel portion are bonded, and water exhaust performance from GDL also shows a difference between the land portion and the channel portion.
As well known in a fuel cell, reaction gases supplied through a flow field of a separator had better be uniformly diffused over the whole area of GDL, and water generated by a reaction had better be promptly exhausted to the outside because water inhibits a chemical reaction from occurring in an electrolyte membrane of a fuel cell.
However, in the case of a typical separator, since the land portion having a large area is bonded to the GDL 12, a diffusion amount of gases varies between a wide GDL area to which the land portion is bonded and a GLD area contacting the flow field of the channel portion.
Referring to FIG. 1, reaction gases (air and hydrogen) are diffused from the flow field of the channel portion to the GDL 12. In such a structure, the diffusion amount of gases inevitably varies between the areas of the GDL 12 that contact the land portion and the channel portion, respectively.
This non-uniformity causes a concentration difference between the area that the channel portion contacts and the area that the land portion contacts among the whole area of the MEA 11 where the electrochemical reaction occurs.
As a result, there occurs a difference of an electrochemical reaction in the whole area of MEA, making it difficult to expect uniform electricity generation over the whole reaction area and reducing the overall performance of a fuel cell.
Also, since a typical separator mainly relies only on diffusion in terms of delivery of reaction gases to GDL, mass transfer from the separator to a catalyst layer is difficult to achieve, which lowers the limiting current density of a fuel cell and thus causes reduction of the overall performance.
Furthermore, since the land portion is bonded at a large area to GDL to which water is exhausted, water that is a by-product of an electrochemical reaction is difficult to exhaust.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.